There is accumulating evidence that in addition to the classic IFN-activated Jak-Stat pathway, non-Stat pathways play important roles in the induction of the biological activities of interferons. We have provided the first evidence that Type I IFNs activate the p38 Map kinase pathway and that activation of this signaling cascade plays a critical role in Type I IFN-dependent transcriptional regulation. Our preliminary studies strongly suggest that the function of p38 MAPK is essential for the generation of IFN-responses, including the suppressive effects of IFNs on human hematopoiesis and the induction of an antiviral state. The overall goal of this proposal is to define the functional role of the p38 pathway in Type I IFN-signaling. The studies of specific aim A will identify upstream elements of the Type I IFN-dependent p38 MAPK pathway and will define the roles of such elements in the generation of IFN-responses. Specific aim B will determine whether the p38 MAPK cascade cooperates with the classic Jak-Stat pathway, or acts independently of it to regulate transcription of interferon-sensitive genes. The potential role that p38-mediated histone serine phosphorylation in the promoters of ISGs may exhibit in Type I IFN-dependent transcriptional activation, will be also examined. Specific aim C will define the role of p38 MAPK in the induction of Type I IFN-dependent antiviral activities against HBV and the generation of growth inhibitory responses on normal bone marrow progenitors. These studies will also identify the stage in the hematopoietic lineage cascade at which p38 is activated by Type I IFNs and will dissect the pathways downstream of p38 that mediate hematopoietic suppression. Specific aim D will determine whether the p38 pathway is activated by IFNalpha in tumor cells from patients suffering from malignant melanoma, and whether defective activation of this pathway accounts for the development of IFN-resistance in melanoma. This multifaceted approach should provide important insights on the mechanisms by which IFN-signals translate to specific biological responses, as well as on the mechanisms by which malignant cells develop resistance to the biological activities of interferons. Understanding such mechanisms may have important translational implications and facilitate the development of novel IFN-based therapeutic approaches in the future.